Method for producing silane

ABSTRACT

The invention relates to a method for producing silane (SiH 4 ) by a) reacting metallurgical silicon with silicon tetrachloride (SiCl 4 ) and hydrogen (H 2 ), to form a crude gas stream containing trichlorosilane (SiHCl 3 ) and silicon tetrachloride (SiCl 4 ), b) removing impurities from the resulting crude gas stream by washing with condensed chlorosilanes, c) condensing and subsequently, separating the purified crude gas stream by distillation, d) returning the partial stream consisting essentially of SiCl 4  to the reaction of metallurgical silicon with SiCl 4  and H 2 , e) disproportionating the partial stream containing SiHCl 3 , to form SiCl 4  and SiH 4  and f) returning the SiH 4  formed by disproportionation to the reaction of metallurgical silicon with SiCl 4  and H 2 , the crude gas stream containing trichlorosilane and silicon tetrachloride being liberated from solids as far as possible by gas filtration before being washed with the condensed chlorosilanes. The washing process with the condensed chlorosilanes is carried out at a pressure of 25 to 40 bar and at a temperature of at least 150° C. in a single-stage distillation column and is carried out in such a way that 0.1 to 3 wt. % of the crude gas stream containing trichlorosilane and silicon tetrachloride is recovered in the form of a condensed liquid phase consisting essentially of SiCl 4 , this liquid phase then being removed from the SiCl 4  circuit and expanded to a pressure of 1 bar outside said SiCl 4  circuit and cooled to a temperature of 10 to 40° C., whereby dissolved impurities separate out and are then removed by filtration.

The present invention relates to a method for producing silane (SiH₄) byreacting metallurgical silicon with silicon tetrachloride (SiCl₄),hydrogen (H₂) and hydrogen chloride (HCl), removing impurities from theresulting crude gas stream containing trichlorosilane (SiHCl₃) anddisproportionating the said SiHCl₃ to form SiCl₄ and silane.

Silane can be used for the manufacture of high-purity silicon asrequired for the manufacture of semi-conductors and solar cells.According to “Silicon for the Chemical Industry IV, Geiranger, Norway,Jun. 3-5, 1998, Ed.: H. A. Øye, H. M. Rong, L Nygaard, G. Schüissler, J.Kr. Tuset, p. 93-112” silane required for the manufacture of high-puritysilicon is produced by two different methods:

Reacting silicon tetrafluoride (SiF₄) with sodium aluminium hydride(NaAlH₄) to form SiH₄ and sodium aluminium fluoride (NaAlF₄), purifyingthe produced SiH₄, separation of high-purity silicon on silicon seedcrystal in a fluidized bed and removal of H₂ from the formed high-puritysilicon granules. Large amounts of NaAl₄ occur in this process whichmust be utilized or marketed accordingly.

Reaction of metallurgical silicon with SiCl₄ and H₂ in a fluidized bedto form SiHCl₃, catalysed two-stage disproportionation of said SiHCl₃ toform SiCl₄ and SiH₄, returning the SiCl₄ formed by disproportionation tothe reaction of metallurgical silicon with SiCl₄ and H₂, thermaldecomposition of the formed SiH₄ on silicon rods to form high-puritysilicon and returning the H₂ formed by decomposition to the reaction ofmetallurgical silicon with SiCl₄ and H₂.

The latter method is characterized in that the inevitable production oflarge amounts of by-products is avoided because the SiCl₄ occurring inthe process is used for the manufacture of SiHCl₃ by reacting said SiCl₄with metallurgical silicon and hydrogen.

Embodiments of said method are specified in “Studies in OrganicChemistry 49, Catalyzed Direct Reactions of Silicon, Elsevier, 1993, p.450 to 457”, DE 3 311 650 C2 und CA-A-1 162 028. According to the abovedocumentation, the manufacture of silane according to the said methodcomprises the following steps:

-   -   1. Reacting of metallurgical silicon with SiCl₄ and H₂ at        temperatures from 400 to 600° C. and a pressure from 20.7 to        41.4 bar in a fluidized-bed reactor.    -   2. Removing impurities, such as not reacted fine silicon, metal        chlorides, polysilane, siloxane and, if necessary, catalyst,        from the resulting reaction mixture containing chlorosilane and        hydrogenous acid by washing the hot crude gas stream with        condensed chlorosilanes.    -   3. Removing the resulting chlorosilane suspension containing        solids.    -   4. Condensing the purified reaction mixture.    -   5. Returning the hydrogen formed in Step 4 in Step 1.    -   6. Separating the purified reaction mixture by distillation to        form SiCl₄ and SiHCl₃.    -   7. Returning the SiCl₄ in Step 1.    -   8. Two-stage catalysed disproportionation of the SiHCl₃ obtained        in Step 6 to form SiH₄ and SiCl₄.    -   9. Returning the SiCl₄ in Step 1.    -   10. Removing impurities by distillation from the SiH₄ obtained        in Step 8.

A disadvantage of the specified method is that the removal of impuritiesfrom the hot gas stream resulting from the reaction in the fluidized bedby washing with condensed chlorosilanes (Step 2) is technically veryexpensive due to the presence of fine solid gas components. There isfurther the risk that the employed apparatus may be choked by solidswhich makes a continuous operation difficult.

The chlorosilane suspension resulting from Step 2 containing siliconmetal and metal chloride is removed in accordance with DE 3 709 577 A1by a specific separation of chlorosilanes and solids by distillationwhereby a high percentage of chlorosilanes can be recovered and isreturned to the circuit. The remaining distillation bottom productcontaining solids and chlorosilane cannot be utilized and must bedisposed of in a way as it is specified, for example, in U.S. Pat. No.4,690,810. This procedure impairs the economic efficiency of the method.Another disadvantage is that together with the recovered chlorosilaneundesired impurities are returned to the process operating state silaneproduction which can result in an undesired concentration of suchimpurities affecting the process.

So characteristic for the method is a returning stream of silicontetrachloride. Combining all relevant equations, silane is produced fromsilicon and hydrogen by this method. Silicon tetrachloride ispermanently circulating during the reaction and does not leave the saidcircuit. $\begin{matrix}{{Si} + {2H_{2}} + {3{SiCl}_{4}}} & \longrightarrow & {4{SiHCl}_{3}} & {\quad(1)} \\{4{SiHCl}_{3}} & \longrightarrow & {{3{SiCl}_{4}} + {SiH}_{4}} & {\quad(2)} \\{{Si} + {2H_{2}}} & \longrightarrow & {SiH}_{4} & {\quad(3)}\end{matrix}$

Because of the incomplete reaction of silicon, hydrogen and silicontetrachloride the first equation should be defined more exact asfollows:Si+(2+x)H₂+(3+y)SiCl₄→4SiHCl₃+xH₂+ySiCl₄  (1a)

This does not change the total result of equation (3), but it becomesvisible that the not reacted silicon tetrachloride increases the circuitstream of silicon tetrachloride.

The object of the present invention was to provide a method for themanufacture of silane that is free of the above specified disadvantagesand allows to produce silane at low costs.

The present invention relates to a method for producing silane (SiH₄) by

-   -   a) reacting metallurgical silicon with silicon tetrachloride        (SiCl₄) and hydrogen (H₂) to form a crude gas stream containing        trichlorosilane (SiHCl₃) and silicon tetrachloride (SiCl₄),    -   b) removing impurities from the resulting crude gas stream by        washing with condensed chlorosilanes to produce a purified crude        gas stream containing trichlorosilane and silicon tetrachloride        and a homogeneous liquid phase consisting essentially of SiCl₄,        this liquid phase then being removed from the circuit,    -   c) condensing and subsequently separating the purified crude gas        stream by distillation to form a partial stream consisting        essentially of SiCl₄ and a partial stream consisting essentially        of SiHCl₃,    -   d) returning the partial stream consisting essentially of SiCl₄        to the reaction of metallurgical silicon with SiCl₄ and H₂,    -   e) disproportionating the partial stream containing        trichlorosilane to form SiCl₄ and SiH₄, and    -   f) returning the SiCl₄ formed by disproportionation to the        reaction of metallurgical silicon with SiCl₄ and H₂,        characterized in that the crude gas stream containing        trichlorosilane and silicon tetrachloride being liberated from        solids as far as possible by gas filtration before being washed        with the condensed chlorosilanes, the washing process with the        condensed chlorosilanes is carried out at a pressure of 25 to 40        bar and at a temperature of at least 150° C. in a multi-stage        distillation column and is carried out in such a way that 0.1 to        3 weight percent of the crude gas stream containing        trichlorosilane and silicon tetrachloride is recovered in the        form of a condensed liquid phase consisting essentially of        SiCl₄, condensed liquid phase consisting essentially of SiCl₄        then being removed from the SiCl₄ circuit and expanded to a        pressure of 1 bar outside said SiCl₄ circuit and cooled to a        temperature of 10 to 40° C., whereby dissolved impurities        separate out and are then removed by filtration.

Preferably washing is carried out in such a way that 0.5 to 1.5 weightpercent of the crude gas stream containing trichlorosilane and silicontetrachloride is recovered as in the form of a condensed liquid phaseconsisting essentially of SiCl₄.

Metallurgical silicon in this meaning refers to silicon containing up toapprox. 3 weight percent iron, 0.75 weight percent aluminium, 0.5 weightpercent calcium and other impurities as can usually be found in siliconobtained by carbothermal reduction of silicon-di-oxide.

Preferably the reaction of metallurgical silicon with SiCl₄ and H₂ (Stepa)) is carried out at temperatures from 500 to 800° C. and a pressurefrom 25 to 40 bar.

Suitable apparatuses for gas filtration are, for example, cyclones orhot-gas filters. In an advantageous embodiment of the method accordingto the invention gas filtration is carried out in several cyclones whichare connected in series or in multi-cyclones. Such filter apparatusesare specified for example in Ullmann's Encyclopedia of IndustrialChemistry, Volume B 2, Unit Operation 1, 5^(th) complete revisedEdition, VCH-Verlagsgesellschaft, Weinheim p. 13-4 to 13-8.Alternatively also hot-gas filters with sintered metal or ceramiccandles or combinations of cyclones and hot-gas filters can be used.Using the above mentioned filter apparatuses ensures that the solids areseparated from the crude gas stream as far as possible enabling anunobstructed subsequent washing with the condensed chlorosilanes. In themethod according to the invention impurities that are still contained inthe crude gas stream after gas filtration, such as metal chlorides,non-metal chlorides, siloxanes and polysilanes, are separated in thecondensed liquid phase consisting essentially of SiCl₄ and can easily beremoved together with it from the process of silane production.

Another advantage is that by this method a solid containing siliconmetal is obtained which can be used in metallurgical processes, such ase.g. the manufacture of iron alloys, due to its high silicon contents.To this end the solid containing silicon metal and metal chloride can bereacted for example with alkaline compounds, such as soda lye, Na₂CO₃,NaHCO₃ and CaO and water, filtered and washed with water to removechloride and dried if necessary.

Preferably the liquid phase consisting essentially of SiCl₄ resultingfrom washing with condensed chlorosilanes and the subsequent pressurereduction and cooling is liberated from impurities separated out bymeans of plate pressure filters. It is preferred to use sintered metals,particularly preferred sintered wire-cloth, as filter elements. Suchfilter elements are commercially available under the trade namesPoroplate® and Fuji-Plate®. Alternatively decanters can also be used toremove the impurities separating out.

The resulting filtrate is excellently suitable as raw material for themanufacture of pyrogenic silicic acid and is therefore preferably usedfor this purpose. Any further reprocessing, e.g. by distillation, is notrequired. The solid resulting from filtration can be inerted in theknown way with alkaline compounds, such as soda lye, Na₂CO₃, NaHCO₃ andCaO and used after inerting as raw material in the manufacture ofcement.

In a particularly preferred embodiment of the method according to theinvention the need of chloride equivalents caused by the discharge offiltrate essentially consisting of SiCl₄ is compensated by adding 0.05to 10 weight percent hydrogen chloride (HCl), based on the amount ofSiCl₄ introduced, as an additional reactand in reacting metallurgicalsilicon with SiCl₄ and H₂. Preferably an amount of 0.5 to 3 weightpercent HCl is used.

Using an amount of 0.5 to 10 weight percent HCl, based on the amount ofSiCl₄ introduced, as additional reactand causes an unexpectedacceleration of the reaction finally resulting in a very high yield ofSiHCl₃, that means high reaction rates near the thermodynamicequilibrium of the SiCl₄ employed, and at the same time high totalyields, i.e. a largely complete utilization of the metallurgical siliconemployed.

Hydrogen chloride is preferably used in an anhydrous form as hydrogenchloride gas.

Hydrogen chloride, for example, cannot be introduced separately in thereactor where the reaction to form trichlorosilane will be carried out.It is also possible, however, to introduce hydrogen chloride in thereactor together with gaseous and/or vaporisable starting materialshydrogen and/or silicon tetrachloride.

The preferred embodiment of the method according to the inventioncomprising the addition of hydrogen chloride when reacting metallurgicalsilicon with SiCl₄ and H₂ is characterized mainly in that anacceleration of the reaction is caused by using the inventive amounts ofHCl as additional reactand thus achieving a higher utilization ratio ofthe metallurgical silicon used and improving the economic efficiency ofthe method considerably.

So the addition of the preferably to be added amount of hydrogenchloride causes a faster activation of silicon. When fresh silicon isreacted with hydrogen and silicon tetrachloride or such reaction iscontinued after an interruption of the process, an induction periodoccurs lasting approx. 100 minutes, for example, in case of a reactiontemperature of 600° C. and a H₂: SiCl₄ mol ratio of 2:1. Such inductionperiod is reduced to 45 minutes by addition of 2 weight percent hydrogenchloride based on silicon tetrachloride and apart from this the sameconditions. Since in a reaction carried out in a continuously operatedfluidized-bed reactor a considerable part of the fluidized bed alwayscontains freshly introduced silicon the faster activation of suchsilicon has an accelerating effect on the process as a whole.

Furthermore, adding hydrogen chloride causes the reaction gasses toattack all over the complete silicon area. When adding solid catalyststo the silicon to be reacted the reaction with the hydrogen/silicontetrachloride gas occurs immediately at the edge of the catalyst cornsresulting in crater-like cavities. Upon further progress of the reactioninto the depth, the silicon surfaces that were previously covered withcatalyst particles are undermined and the particles disengage from thesilicon corn and are subsequently carried away as small particles fromthe fluidized bed. Thus neither the said silicon particles which werecarried away, nor the adherent catalysts are available for the desiredreaction. The consequences are a worse total yield and a decreasingreaction velocity of the silicon particle depleted of catalyst. The sameoccurs, in principle, in the uncatalysed reaction of silicon withhydrogen/silicon tetrachloride without addition of hydrogen chloride. Inthis case the reaction proceeds in crater-like cavities along the bandformed by the impurities separating out. Such band contain theimpurities contained in silicon or introduced by the raw materials andthe production process, essentially iron, aluminium, calcium, titanium,causing also an acceleration of the reaction. In contrast, the reactionof silicon with hydrogen/silicon tetrachloride in the presence ofhydrogen chloride occurs all over the surface of the silicon cornsforming a high number of crater-like cavities on the surface of suchsilicon corns. Since the complete surface of the silicon corns providesessentially more reaction area than the sections occupied by catalystparticles and/or the bands on the edges of the corns containing thecollected impurities, the silicon area participating in the reaction ismuch bigger thus causing an acceleration of the reaction velocitydepending on the area.

The preferred embodiment of the method according to the inventioncomprising the addition of hydrogen chloride when reacting metallurgicalsilicon with SiCl₄ and H₂ is also characterized in that the amount ofsilicon carried out undesiredly is reduced. Reacting on large surfacesof the silicon corns in the presence of hydrogen chloride prevents theundermining of the spots containing catalyst and the corn-burstingtrenching reaction along the bands containing impurities preventing theblasting off of small silicon particles and their being carried out ofthe fluidized bed by the reaction gasses. This increases the yield oftrichlorosilane based on the amount of silicon tetrachloride used aswell as based on the silicon used.

In addition to this, adding hydrogen chloride results in a constantreaction velocity in the course of the decomposition of silicon.Contrary to the catalysed reaction without addition of hydrogen chloridethe reaction velocity of the silicon/hydrogen/silicon tetrachloridereaction is not reduced considerably in the presence of hydrogenchloride. This finding is surprising, since the silicon corns aregetting smaller in the course of the reaction which should cause areduction of surface of a given amount of silicon, and thesurface-related residence time of the reaction gasses decreases as well.

The selection of the reactor for the reaction according to the inventionis not critical, provided that under the reaction conditions the reactorshows adequate stability and permits the contact of the startingmaterials. The process can be carried out, for example, in a fixed bedreactor, a rotary tubular kiln or a fluidized-bed reactor. It ispreferred to carry out the reaction in a fluidized-bed reactor.

The material of the reactor must resist the reaction conditionsmentioned for SiHCl₃ synthesis. The requirements on the resistance ofthe construction materials of the reactor apply also for any precedingand secondary parts of the plant, such as for example cyclones or heatexchangers. These requirements are fulfilled, for example, by nickelbase alloys.

Further acceleration of the reaction of metallurgical silicon withSiCl₄, H₂ and HCl, if applicable, can be achieved by the use ofcatalysts. Particularly suitable catalysts are copper, iron, copper oriron compounds or any mixtures thereof.

Surprisingly it was found that the catalysts unfold particularly highefficiency when the metallurgical silicon is provided in a milled formand was mixed intensively with the catalysts prior to the reaction.

It is therefore preferred in the method according to the invention tocarry out the reaction to form trichlorosilane (Step a)) in the presenceof catalyst, and to mix the metallurgical silicon intensively with thecatalysts prior to the reaction.

Preferably the silicon is provided in fine form, particularly preferredwith an average grain diameter of 10 to 1000 μm, more particularlypreferred of 100 to 600 μm. The average grain diameter is calculated asthe arithmetical mean of the values determined in a sieve analysis ofthe silicon.

Preferably, the mixing of catalyst and silicon is carried out inapparatuses ensuring a very intensive mixing. Particularly suitable forthis purpose are mixers provided with rotary mixing tools. Such mixersare specified for example in “Ullmann's Encyclopedia of IndustrialChemistry, Volume B2, Unit Operations I, p.27-1 to 27-16, VCHVerlagsgesellschaft, Weinheim”. Particularly preferred is the use ofplough blade mixers.

During intensive mixing the catalyst can be milled further which resultsin a very good distribution of the catalyst during mixing and a verygood adherence of the catalyst on the silicon surface. Therefore alsocatalysts can be used which are not available in a very fine form orcannot be milled to the required fineness.

In the case of insufficient mixing a large portion of catalyst isdirectly carried out of the fluidized bed together with the gaseousreactands and/or products due to poor adherence of catalyst to siliconparticles and is therefore not available for the reaction any more. Thiscauses an increased demand for catalyst impairing the economicefficiency of the method. This is avoided by intensive mixing of siliconand catalyst.

Preferably the time for mixing silicon and catalyst is 1 to 60 minutes.As a rule, longer mixing times are not required. Particularly preferredare mixing times from 5 to 20 minutes.

Intensive mixing of catalyst and silicon can be carried out for examplein an inert atmosphere or in the presence of hydrogen or other gasseswith a time-reducing effect, e.g. carbon monoxide. This preventsformation of an oxidic layer on the individual silicon particles. Suchlayer prevents direct contact between catalyst and silicon which wouldresult in a poorer catalysing of the reaction with silicontetrachloride, hydrogen and, if necessary, hydrogen chloride totrichlorosilane.

An inert atmosphere can be achieved, for example, by adding an inert gasduring mixing. Suitable inert gasses are, for example, nitrogen and/orargon.

Particularly preferred is the mixing of silicon and catalyst in thepresence of hydrogen.

On principle, all catalysts known for reacting silicon with silicontetrachloride, hydrogen and, if necessary, hydrogen can be used ascatalyst.

Particularly suitable catalysts are copper and iron catalysts. Examplesfor this are copper oxide catalysts (e.g. Cuprokat®, manufacturer:Norddeutsche Affinerie), copper chloride (CuCl CuCl₂), copper metal,iron oxides (e.g. Fe₂O₃, Fe₃O₄), ferrous chlorides (FeCl₂, FeCl₃) andtheir mixtures.

Preferred catalysts are copper oxide catalysts and iron oxide catalysts.

Particularly when using copper oxide catalysts and iron oxide catalystsit has proved advantageous to mix the silicon at a temperature from 100to 400° C., preferably from 130 to 250° C. This removes any moistureresidue adherent to catalysts which would otherwise have a negativeimpact on the reaction of silicon with SiCl₄, H₂ and HCl, if applicable.Mixing in the presence of reducing gasses, preferably hydrogen,furthermore reduces oxidic components of the catalysts preventing ayield reduction caused by oxygen or oxides when reacting metallurgicalsilicon with SiCl₄ and H₂. Furthermore a better adherence of catalyst tothe silicon surface is achieved by this method avoiding largely anycatalyst loss in the fluidized bed.

It is also possible to use mixtures of copper catalysts and/or ironcatalysts with further catalytically active components. Suchcatalytically active components are, for example, metal halogenides,such as e.g. chlorides, bromides or iodides of aluminium, vanadium orantimony.

Preferably the amount of catalyst used, calculated as metal, is 0.5 to10 weight percent, particularly preferred 1 to 5 weight percent, basedon the silicon employed.

The mol ratio of hydrogen to silicon tetrachloride when reactingmetallurgical silicon with SiCl₄ und H₂ can be for example 0.25:1 to4:1. A mol ratio of 0.6:1 to 2:1 is preferred.

The partial stream resulting from the separation of the purified crudegas stream containing trichlorosilane and silicon tetrachloride bydistillation consisting essentially of SiHCl₃ is disproportionatedpreferably in a column at a pressure from 1 to 10 bar, wherein thecolumn provides at least two reactive/destillative reaction zones.

Such disproportionation is carried out on catalytically active solids,preferably in catalyst beds each one consisting of a layer of bulkmaterial of said catalytically active solids which can be streamedthrough by the products of the disproportionation. Instead of a layer ofbulk material also packed catalyst bodies can be provided in thereaction zone.

Suitable catalytically active solids are known and specified, forexample, in DE 2 507 864 A1. Such suitable solids, for example, aresolids carrying amino groups or alkyleneamino groups on a structure ofpolystyrene, crosslinked with divinylbenzole. Suitable amino groups oralkylenamino groups are for example: dimethylamino, diethylamino,ethylmethylamino, di-n-propylamino, di-iso-propylamino,di-2-chlorethylamino, di-2chlorpropylamino groups and the respectivehydrochlorides, or the trialkylammonium groups formed from them bymethylation, ethylation, propylation, butylation, hydroxyethylation orbenzylation with chloride as counterion. Of course, in the case ofquaternary ammonia salts or protonized ammonia salts also catalyticallyactive solids with other anions, e.g. hydroxide, sulphate, bisulphate,bicarbonate etc. can be introduced into the method according to theinvention, a transformation into the chloride form, however, isinevitable under the reaction conditions in the course of time, thisapplies also to organic hydroxy groups. Therefore, ammonia saltscontaining chloride as counterion are preferred.

Also those solids are suitable as catalytically active solids whichconsist of a structure of polyacrylic acid, particularly apolyacrylamide structure, that has bound, for example,trialkylbenzylammonium via an alkyl group.

Another suitable group of catalytically active solids are, for example,solids carrying sulphonate groups on a structure of polystyrene,cross-linked with divinylbenzole, which are opposed by tertiary orquaternary ammonium groups as cations.

As a rule, macroporous or mesoporous ion exchangers are more suitablethan gel resins.

Preferably the method according to the invention is integrated into ageneral method for producing hyper-pure silicon.

Particularly preferred, the method according to the invention isintegrated into a multistage general method for producing hyper-puresilicon, as specified for example in “Economics of Polysilicon Process,Osaka Titanium Co., DOE/JPL 1012122 (1985), 57-78” and comprising thefollowing steps:

-   -   a) Production of trichlorosilane;    -   b) Disproportionation of trichlorosilane to yield silane;    -   c) Purifying silane to obtain high-purity silane, and    -   d) Thermal decomposition of silane in a fluidized-bed reactor        and depositing of hyper-pure silicon on the silicon particles        which form the fluidized bed.

The particular advantages of adding hydrogen chloride when reactingmetallurgical silicon with SiCl₄ and H₂, as it is carried out in Step a)of a preferred embodiment of the method according to the invention, isbeing explained in more detail in the following examples. The examplesshall not be understood, however, as a restriction to the inventive ideainsofar.

EXAMPLE 1A

400 g silicon (99.3 weight percent silicon, average diameter ofparticles Dp=250-315 μm) were provided in a fluidized-bed reactor withan internal diameter (I.D.) of 0.05 m and were reacted with hydrogen andsilicon tetrachloride at a temperature T=600° C. and a total pressure ofP_(tot)=1.1 bar. The mol ratio H₂/SiCl₄ was 2 in the presence of 20volume percent N₂. The reaction was carried out under addition of 2weight percent HCl, based on the amount of silicon tetrachloride. Thetime for achieving 95% of stationary yield of trichlorosilane wasT_(95%)=45 min.

EXAMPLE 1b (COMPARATIVE EXAMPLE)

The reaction according to Example 1a was repeated, however, withoutadding HCl. The time for achieving 95% of stationary yield oftrichlorosilane was T_(95%)=100 min.

EXAMPLE 2a

400 g silicon (99.3 weight percent silicon, average diameter ofparticles Dp=250-315 μm) were provided in a fluidized-bed reactor withan internal diameter (I.D.) of 0.05 m and were reacted with hydrogen andsilicon tetrachloride at a temperature T=600° C. and a total pressure ofP_(tot)=1.1 bar. The mol ratio H₂/SiCl₄ was 2 in the presence of 20volume percent N₂. The reaction was carried out several days. An amountof 1.5±1-0.5 weight percent based on the amount of silicon tetrachloridewas added continuously. After 24.2% of silicon were reacted the reactionmass was examined by means of surface electron microscopy. It shows thata reaction of silicon had occurred all over the surface of the siliconparticles.

EXAMPLE 2b (COMPARATIVE EXAMPLE)

The reaction according to Example 2a was repeated, this time, however,without adding hydrogen chloride. After 23.4% of silicon were reactedthe reaction mass was examined by means of surface electron microscopy.It can be recognized that the silicon reacted only on single points andedges.

EXAMPLE 2c (COMPARATIVE EXAMPLE)

Similar to Example 2a, 400 g of silicon (99.3 weight percent silicon,average diameter of particles Dp=160-195 μm) were provided in afluidized-bed reactor with an internal diameter (I.D.) of 0.05 m andwere reacted with hydrogen and silicon tetrachloride at a temperatureT=600° C. and a total pressure of P_(tot)=1.1 bar. The mol ratioH₂/SiCl₄ was 2 in the presence of 20 volume percent N₂. The reaction wascarried out several days. But no HCl was added, the reaction was carriedout instead in the presence of 1 weight percent Cu in form of Cumetal/Cu₂O/CuO as catalyst. After 31.4% of silicon were reacted thereaction mass was examined by means of surface electron microscopy. Itshows that a slight reaction of silicon occurred, but also underminingof the surface and formation of big cavities can be observed.

EXAMPLE 3a

Example 2 a was repeated, but this time the reaction was not interruptedafter 24.2% of silicon had reacted. The amount of material carried awaywas determined by means of the amounts separated in a cyclone. Theamount of material carried away decreased continuously according to thedegree of reacted silicon and was below 0.5 weight percent based on theamount of reacted silicon already when a degree of 15% of reactedsilicon was reached.

EXAMPLE 3b (COMPARATIVE EXPERIMENT)

Example 2 b was repeated, but this time the reaction was not interruptedafter 23.4% of silicon had reacted. The amount of material carried awaywas determined by means of the amounts separated in a cyclone. Theamount of material carried away increased up to a degree of reactedsilicon of approx. 15% and then decreased. At a degree of reactedsilicon of approx. 15% the amount of material carried away was above 1.0weight percent based on the amount of reacted silicon.

EXAMPLE 3c (COMPARATIVE EXPERIMENT)

Example 2 c was repeated, but this time the reaction was not interruptedafter 31.4% of silicon had reacted. The amount of material carried awaywas determined by means of the amounts separated in a cyclone. Theamount of material carried away increased continuously according to thedegree of reacted silicon and was above 1.0 weight percent based on theamount of reacted silicon already when a degree of 15% of reactedsilicon was reached. At a degree of reacted silicon of approx. 45%amounts of material carried away of above 7.0 weight percent based onthe amount of reacted silicon were observed.

A comparison of the amount carried away in a reaction according toExamples 3a, 3b and 3c is depicted in FIG. 1 specifying the amountcarried away (A) based on the amount of reacted silicon in weightpercent in comparison with the degree of reacted silicon (X) in %. Thedesignations 3a, 3b, 3c of the graphs correspond to the numbering of theexamples.

EXAMPLE 4

400 g silicon (99.3 weight percent silicon, average diameter ofparticles (Dp) in uncatalysed reaction and under addition of HCl:Dp=250-315 μm; Cu-catalysed reaction: Dp=160-195 μm) were provided in afluidized-bed reactor with an internal diameter (I.D.) of 0.05 m and thehydrogen-chloride reaction was carried out at a temperature T=600° C.and a total pressure of P_(tot)=1.1 bar for several days. The mol ratioH₂/SiCl₄ was 2 in the presence of 20 volume percent N₂. Three forms ofreaction were carried out: a) not catalysed without adding HCl(comparison), b) Cu-catalysed without adding HCl (1% Cu as Cumetal/Cu₂O/CuO) (comparison) and c) without Cu catalyst and withadditional adding of 1.5±0.5 weight percent of HCl. In the differentforms of the reaction the yield of the target product trichlorosilanewas determined. It shows that the yield in a reaction according to theinvention including the addition of HCl is not decreasing asconsiderably during the reaction as in the case of the uncatalysed or Cucatalysed reactions without addition of HCl.

1. Method for producing silane (SiH₄) by a) reacting metallurgicalsilicon with silicon tetrachloride (SiCl₄) and hydrogen (H₂) to form acrude gas stream containing trichlorosilane (SiHCl₃) and silicontetrachloride (SiCl₄), b) removing impurities from the resulting crudegas stream by washing with condensed chlorosilanes to produce a purifiedcrude gas stream containing trichlorosilane and silicon tetrachlorideand a homogeneous liquid phase consisting essentially of SiCl₄, thisliquid phase then being removed from the circuit, c) condensing andsubsequently separating the purified crude gas stream by distillation toform a partial stream consisting essentially of SiCl₄ and a partialstream consisting essentially of SiHCl₃, d) returning the partial streamconsisting essentially of SiCl₄ to the reaction of metallurgical siliconwith SiCl₄ and H₂, e) disproportionating the partial stream containingSiHCl₃ to form SiCl₄ and SiH₄, and f) returning the SiCl₄ formed in thereaction to the reaction of metallurgical silicon with SiCl₄ and H₂,characterized in that the crude gas stream containing trichlorosilaneand silicon tetrachloride is liberated from solids as far as possible bygas filtration before being washed with the condensed chlorosilanes, thewashing process with the condensed chlorosilanes is carried out at apressure of 25 to 40 bar and at a temperature of at least 150° C. in amulti-stage distillation column and is carried out in such a way that0.1 to 3 weight percent of the crude gas stream containingtrichlorosilane and silicon tetrachloride is recovered in the form of acondensed liquid phase consisting essentially of SiCl₄, such condensedliquid phase consisting essentially of SiCl₄ is removed from the SiCl₄circuit and expanded to a pressure of 1 bar outside said SiCl₄ circuitand cooled to a temperature of 10 to 40° C., whereby dissolvedimpurities separate out, and such impurities separating out are removedby filtration.
 2. A method according to claim 1, characterized in thatthe gas filtration is carried out in several cyclones which areconnected in series.
 3. A method according to claim 1, characterized inthat the impurities separating out are removed from the liquid phaseconsisting essentially of SiCl₄ by filtration by means of plate pressurefilters provided with sintered wire-cloth as filter element.
 4. A methodaccording to claim 3, characterized in that the filtrate consistingessentially of SiCl₄ is used as raw material for the manufacture ofpyrogenic silicic acid.
 5. A method according to claim 1, characterizedin that 0.05 to 10 weight percent hydrogen chloride (HCl), based on theamount of SiCl₄, is used as an additional reactant in reactingmetallurgical silicon with SiCl₄ and H₂.
 6. A method according to claim1, characterized in that 0.5 to 3 weight percent HCl, based on theamount of SiCl₄ introduced, is used as an additional reactant inreacting metallurgical silicon with SiCl₄ and H₂.
 7. A method accordingto claim 1, characterized in that the hydrogen chloride to be added isused in an anhydrous form as hydrogen chloride gas.
 8. A methodaccording to claim 1, characterized in that the reaction ofmetallurgical silicon with SiCl₄ and H₂ is carried out in the presenceof a catalyst.
 9. A method according to claim 8, characterized in thatthe catalysts used are at least one of copper, iron, copper compounds,iron compounds or any mixtures thereof.
 10. A method according to claim1, characterized in that the reaction of metallurgical silicon withSiCl₄ and H₂ is carried out at temperatures from 500 to 800° C. and apressure from 25 to 40 bar.
 11. A method according to claim 1,characterized in that the gas filtration is carried out in severalcyclones which are connected in one multi-cyclone.